An aim of the invention is to make a digital filtering apparatus which is modular, which can be parameterized, and which is suited to installation in custom-designed circuits, e.g. of the ASIC (Application Specific Integrated Circuit) type, over an input bandwidth of several megahertz (e.g. 6 MHz) with a high-performance filtering characteristic. The apparatus must be programmable in passband, in transition band, in central frequency, and in type (i.e. low-pass, high-pass, band-pass, band-stop, or multi-band). Moreover, the apparatus must satisfy space requirements, in particular relating to: reliability, optimizing consumption, compactness, and insensitivity to radiation.
There are two possible approaches to making such apparatus:
It is possible to use a bank of filter elements, with the input band being decomposed by the bank into sub-band elements, and with only those sub-bands that belong to the band of the desired filter being recombined by the bank. The filter elements isolating the sub-bands are then fixed, and programmability is provided by selecting which sub-bands to recombine. The filter elements have filter characteristics and stability specifications which are quite tight: two types of installations are possible: one using surface acoustic wave (SAW) type filters; and the other using digital filters. But such "bank-of-filters" architecture is entirely satisfactory only for specific applications, since the band of the resulting filter can be programmed only at a pitch equal to the width of one sub-band. Moreover, great care must be taken in making the filter elements: their frequency stability and their filter characteristics must provide continuity in phase and in amplitude at the boundaries between the sub-bands so that the final recombination can be performed under the best possible conditions. Furthermore, frequency resolution is limited by the number of filter elements.
It is also possible to use a filter having a programmable filter characteristic. There are two main types of such programmable filters: switched-capacitance filters, and digital filters. Both types may be made with very large scale integration (VLSI) components available on the market. Switched-capacitance filters can process only signals having frequencies lower than a few tens of kilohertz. As regards variable-characteristic digital filters, the VLSI-type components on the market are at the edge of the desired frequency range. Furthermore, since they are general-purpose, they are over-dimensioned in most applications in particular as regards the size of the digital words processed, and this leads to an excessive consumption cost which is detrimental in a space application. Such VLSI digital filters integrate a large number of multiplier-adders which are themselves complex functions whose reliability is critical: since the architecture does not provide internal redundancy, reliability must be improved by adding on additional components, which is detrimental to the consumption of the filter. Moreover, each new type of component needs to be individually qualified for a space application.
Since no other known component is capable of solving the problem posed, an aim of the invention is to propose a solution to the problem, which solution enables the above-listed drawbacks to be mitigated.